Recent studies of thick filament structure in skeletal muscle indicate that a simple helical model will not suffice to explain observations. In partiular, optical diffraction patterns obtained from the banded region of thick filaments differ from those obtained from the non-banded region in significant ways. Concurrent studies of synthetic myosin filaments indicate a hiher degree of organization than had previously been suggested, the parameters similar to those of natural filaments. When synthetic myosin filaments are prepared with C-protein, however, the axial repeat changes greatly. Synthetic myosin filaments, then, provide a model system for the study of interactions of thick filament proteins and the closest in vitro analogue presently available. I propose to study thick filament structure by further investigation of the banded and non-banded regions of the natural filament and by analysis of thick filament protein interactions in artificial systems. The nature of the aggregation process for myosin and myosin rods, the effects of C-protein on this aggregation and resultant structures, will be studied and related to natural filament observations.